Process for synthesizing trans-1,4-polybutadiene

ABSTRACT

The process and catalyst system of this invention can be utilized to synthesize polybutadiene rubber having a high trans content and a low melting point by solution polymerization. The trans-polybutadiene rubber made by the process of this invention can be utilized in tire tread rubbers that exhibit outstanding wear characteristics. More importantly, the trans-polybutadiene rubber of this invention can be easily processed because of its low level of crystallinity. In fact, the trans-polybutadiene made by the process of this invention does not need to be heated in a “hot-house” before being used in making rubber compounds. This invention more specifically reveals a process for synthesizing trans-polybutadiene rubber which comprises polymerizing 1,3-butadiene in an organic solvent in the presence of a catalyst system which comprises (a) an organolithium compound, (b) a barium compound selected from the group consisting of (i) barium salts of cyclic alcohols, such as barium mentholate, and (ii) barium thymolate, and (c) an organoaluminum compound. The trans-polybutadiene made with the catalyst system of this invention typically has a glass transition temperature that is within the range of about -97° C. to about -90° C., a melting point that is within the range of about -30° C. to about 30° C., and a number average molecular weight that is within the range of about 50,000 to about 400,000.

This application claims the benefit of United States Provisionalapplication Ser. No. 60/174,151 filed on Dec. 31, 1999.

BACKGROUND OF THE INVENTION

By virtue of its high level of crystallinity, trans-1,4-polybutadiene(TPBD) is typically a thermoplastic resin. Because it contains manydouble bonds in its polymeric backbone, TPBD can be blended and cocuredwith rubber. TPBD is similar to syndiotactic-1,2-polybutadiene in thisrespect. Even though trans-1,4-polybutadiene having a high melting pointis a thermoplastic resin, it becomes elastomeric when cured alone orwhen cocured with one or more rubbers.

Good molecular weight control can normally be achieved by utilizing ananionic polymerization system to produce TPBD. There is typically aninverse relationship between the catalyst level utilized and themolecular weight attained when anionic polymerization systems are used.Such an anionic polymerization system is disclosed in U.S. Pat. No.4,225,690. The catalyst system disclosed therein is based on adialkylmagnesium compound which is activated with a potassium alkoxide.However, such catalyst systems have not proven to be commerciallysuccessful.

TPBD is normally prepared utilizing transition metal catalysts or rareearth catalysts. The synthesis of TPBD with transition metal catalystsis described by J. Boor Jr., “Ziegler-Natta Catalysts andPolymerizations,” Academic Press, New York, 1979, Chapters 5-6. Thesynthesis of TPBD with rare earth catalysts is described by D. K.Jenkins, Polymer, 26, 147 (1985). However, molecular weight control isdifficult to achieve with such transition metal or rare earth catalystsand monomer conversions are often very modest.

Japanese Patent Application No. 67187-1967 discloses a catalyst systemand technique for synthesizing TPBD consisting of 75 to 80 percenttrans-1,4-structure and 20 to 25 percent 1,2-structure. The catalystsystem described by this reference consists of a cobalt compound havinga cobalt organic acid salt or organic ligand, an organoaluminum compoundand phenol or naphthol. Gel formation is a serious problem that isfrequently encountered when this three-component catalyst system isutilized in the synthesis of TPBD. Gelation is a particularly seriousproblem in continuous polymerizations. By utilizing this catalyst systemand technique, TPBD can be synthesized in a continuous process with onlyminimal amounts of gel formation.

U.S. Pat No. 5,089,574 is based upon the finding that carbon disulfidewill act as a gel inhibitor in conjunction with three component catalystsystems which contain an organocobalt compound, an organoaluminumcompound and a para-alkyl substituted phenol. U.S. Pat. No. 5,089,574also indicates that conversions can be substantially improved byutilizing para-alkyl substituted phenols which contain from about 12 toabout 26 carbon atoms and which preferably contain from about 6 to about20 carbon atoms.

U.S. Pat. No. 5,089,574 more specifically reveals a process forsynthesizing trans-1,4-polybutadiene in a continuous process whichcomprises continuously charging 1,3-butadiene monomer, an organocobaltcompound, an organoaluminum compound, a para-substituted phenol, carbondisulfide and an organic solvent into a reaction zone; allowing the1,3-butadiene monomer to polymerize in said reaction zone to form thetrans-1,4-polybutadiene; and continuously withdrawing thetrans-1,4-polybutadiene from said reaction zone.

U.S. Pat. No. 5,448,002 discloses that dialkyl sulfoxides, diarylsulfoxides and dialkaryl sulfoxides act as molecular weight regulatorswhen utilized in conjunction with cobalt-based catalyst systems in thepolymerization of 1,3-butadiene monomer into TPBD. U.S. Pat. No.5,448,002 reports that the molecular weight of the TPBD produceddecreases with increasing levels of the dialkyl sulfoxide, diarylsulfoxide or dialkaryl sulfoxide present as a molecular weightregulator.

U.S. Pat. No. 5,448,002 specifically discloses a process for thesynthesis of trans-1,4- polybutadiene which comprises polymerizing 1,3-butadiene monomer under solution polymerization conditions in thepresence of at least one sulfoxide compound selected from the groupconsisting of dialkyl sulfoxides, diaryl sulfoxides and dialkarylsulfoxides as a molecular weight regulator and in the presence of acatalyst system which includes an organocobalt compound, anorganoaluminum compound and a para-alkyl substituted phenol.

The presence of residual cobalt in TPBD made with cobalt-based catalystsystems is not desirable. This is because the residual cobalt acts as aprooxidant leading to polymer instability during storage. This is aparticular problem in cases where the TPBD is stored in a “hothouse”prior to usage, which is a standard procedure in many industries, suchas the tire industry. In any case, high levels of residual cobalt in theTPBD lead to problems with polymer stability.

Unfortunately, carbon disulfide is typically required as a gel-reducingagent in the synthesis of TPBD with cobalt-based catalyst systems. Thisis particularly true in the case of continuous polymerization systems.However, the presence of carbon disulfide in such systems reduces thelevel of catalyst activity and generally makes it necessary to increasethe level of cobalt in the catalyst system. Thus, in cases where carbondisulfide is required for gel control, the level of cobalt needed isfurther increased. This accordingly leads to greater polymerinstability.

Due to its high melting point, it is normally necessary to heat TPBD inorder for it to be processed using conventional mixing equipment, suchas a Banbury mixer or a mill mixer. This heating step is typicallycarried out by storing the trans-1,4-polybutadiene in a “hot-house” fora few days prior to its usage. During this storage period, the bails ofthe polymer are slowly heated to a temperature above about 104° F. (40°C.). At such temperatures, the polymer can be readily processed instandard mixing equipment. However, the TPBD typically undergoesundesirable oxidative crosslinking which leads to gelation during thislong heating period. This oxidation can crosslink the TPBD to such ahigh degree that it cannot be processed utilizing standard mixingtechniques. In other words, the oxidative gelation that occurs candestroy the polymer.

U.S. Pat. No. 5,854,351 discloses that TPBD which contains a processingoil can be rapidly heated by radio frequency electromagnetic radiation.The radio frequency waves used in such a heating process typically havea frequency that is within the range of about 2 to 80 MHz (megahertz).By utilizing such a technique, an 80-pound (30 kg) bail of TPBD can berapidly heated to a temperature above 104° F. (40° C.) in a matter ofminutes. During this rapid heating process, oxidative gelation does notoccur to a significant degree. This is, of course, in contrast toconventional heating techniques where bails of TPBD are slowly warmed byconvection heating to the required temperature over a period of days.During this long heating period, the TPBD undergoes highly undesirableoxidative crosslinking.

U.S. Pat. No. 5,854,351 more specifically discloses a technique formixing trans-1,4- polybutadiene with at least one rubbery polymer whichcomprises: (1) heating the trans-1,4-polybutadiene to a temperaturewhich is within the range of 105° F. (41° C.) to 200° F.(93° C.) byexposing it to electromagnetic radiation having a frequency in the rangeof about 2 MHz to about 80 MHz, wherein the trans-1,4-polybutadiene isoil-extended with at least 10 phr of a processing oil; and (2) mixingthe trans-1,4-polybutadiene with said rubbery polymer before any portionof the trans-1,4-polybutadiene cools to a temperature below 104 F.° (41°C.).

U.S. Pat. No. 5,100,965 discloses a process for synthesizing a hightrans polymer which comprises adding (a) at least one organolithiuminitiator, (b) an organoaluminum compound, (c) a barium alkoxide and (d)a lithium alkoxide to a polymerization medium which is comprised of anorganic solvent and at least one conjugated diene monomer.

U.S. Pat. No. 5,100,965 further discloses that high trans polymers canbe utilized to improve the characteristics of tire tread rubbercompounds. By utilizing high trans polymers in tire tread rubbercompounds, tires having improved wear characteristics, tear resistanceand low temperature performance can be made.

In commercial applications where recycle is required, the use of bariumalkoxides can lead to certain problems. For instance, barium t-amylatecan react with water to form t-amyl alcohol during steam-stripping inthe polymer finishing step. Since t-amyl alcohol forms an azeotrope withhexane, it co-distills with hexane and thus contaminates the feedstream.

SUMMARY OF THE INVENTION

This invention is based upon the discovery that the problem of recyclestream contamination can be solved by synthesizingtrans-1,4-polybutadiene utilizing a catalyst system which is comprisedof (a) an organolithium compound, (b) a barium compound selected fromthe group consisting of (i) barium salts of cyclic alcohols, such asbarium mentholate, and (ii) barium thymolate, and (c) an organoaluminumcompound. The problem of recycle stream contamination is solved byutilizing a barium salt of a cyclic alcohol as the barium compound inthe catalyst system. Barium mentholate is highly preferred because itdoes not co-distill with hexane or form compounds during steam-strippingwhich co-distill with hexane. Since the boiling points of the cyclicalcohols generated upon the hydrolysis of their metal salts are veryhigh, they do not co-distill with hexane and contaminate recyclestreams. Additionally, such cyclic alcohols are considered to beenvironmentally safe. In fact, menthol (the hydrolyzed product of bariummentholate) is commonly used as a food additive.

The trans-1,4-polybutadiene made with such barium containing catalystsystems has a melting point that is within the range of about -30° C. to+30° C. Because the trans-1,4-polybutadiene synthesized with thecatalyst system of this invention has a high melting point it does notneed to be heated in a “hot-house” before it is blended with otherrubbery polymers or utilized in making rubber products, such as tires.Additionally, the trans-1,4-polybutadiene is strain crystallizable andcan be employed in manufacturing tire tread compounds that exhibit wearcharacteristics. The trans-1,4-polybutadiene also typically has a glasstransition temperature which is within the range of about -97° C. toabout −90° C., a number average molecular weight which is within therange of about 50,000 to about 400,000, and a Mooney ML 1+4 viscositywhich is within the range of about 5 to about 110.

The present invention more specifically discloses a process forsynthesizing trans-1,4-polybutadiene which comprises polymerizing1,3-butadiene monomer in the presence of a catalyst system which iscomprised of (a) an organolithium compound, (b) a barium compoundselected from the group consisting of (i) a barium salt of a cyclicalcohol, and (ii) barium thymolate, and (c) an organoaluminum compound.

The present invention further discloses a process for synthesizingtrans-1,4-polybutadiene which comprises polymerizing 1,3-butadienemonomer in the presence of a catalyst system which is comprised of (a)an organolithium compound, (b) a barium compound selected from the groupconsisting of (i) a barium salt of a cyclic alcohol, and (ii) bariumthymolate, (c) an organoaluminum compound, and (d) a lithium salt of acyclic alcohol.

DETAILED DESCRIPTION OF THE INVENTION

The polymerizations of the present invention will normally be carriedout in a hydrocarbon solvent that can be one or more aromatic,paraffinic, or cycloparaffinic compounds. These solvents will normallycontain from 4 to 10 carbon atoms per molecule and will be liquid underthe conditions of the polymerization. Some representative examples ofsuitable organic solvents include pentane, isooctane, cyclohexane,methylcyclohexane, isohexane, n-heptane, n-octane, n-hexane, benzene,toluene, xylene, ethylbenzene, diethylbenzene, isobutylbenzene,petroleum ether, kerosene, petroleum spirits, petroleum naphtha, and thelike, alone or in admixture.

In the solution polymerizations of this invention, there will normallybe from 5 to 30 weight percent monomer (1,3-butadiene) in thepolymerization medium. Such polymerization media are, of course,comprised of the organic solvent and the monomer. In most cases, it willbe preferred for the polymerization medium to contain from 10 to 25weight percent monomer. It is generally more preferred for thepolymerization medium to contain 15 to 20 weight percent monomer.

The trans-1,4-polybutadiene made utilizing the catalyst system andtechnique of this invention are comprised of repeat units that arederived from 1,3-butadiene. The trans-1,4-polybutadiene typically has atrans-microstructure content of about 60% to about 80%. Thetrans-1,4-polybutadiene made in accordance with this invention exhibitsa low polydispersity. The ratio of the weight average molecular weightto the number average molecular weight of such trans-l,4-polybutadienewill typically be less than 1.5. It is more typical for the ratio of theweight average molecular weight to the number average molecular weightof the trans-1,4-polybutadiene to be less than about 1.3. It is normallypreferred for the high trans-1,4-polybutadiene of this invention to havea ratio of weight average molecular weight to number average molecularweight which is less than about 1.2.

The trans-1,4-polybutadiene made in accordance with this invention willtypically have a melting point which is within the range of about -20°C. to about 40° C. They also typically have a glass transitiontemperature that is within the range of about -97° C. to about -90° C.

The polymerizations of this invention are initiated by adding anorganolithium initiator, an organoaluminum compound, and a barium saltof a cyclic alcohol to a polymerization medium containing the1,3-butadiene monomer. Preferably, the polymerizations of this inventionare initiated by adding an organolithium initiator, an organoaluminumcompound, a barium salt of a cyclic alcohol, and a lithium salt of acyclic alcohol. Such polymerization can be carried out utilizing batch,semi-continuous or continuous techniques.

The organolithium initiators employed in the process of this inventioninclude the monofunctional and multifunctional types known forpolymerizing the monomers described herein. The multifunctionalorganolithium initiators can be either specific organolithium compoundsor can be multifunctional types which are not necessarily specificcompounds but rather represent reproducible compositions of regulablefunctionality.

The amount of organolithium initiator utilized will vary with themolecular weight that is desired for the trans-1,4-polybutadiene beingsynthesized. However, as a general rule from 0.01 to 1 phm (parts per100 parts by weight of monomer) of an organolithium initiator will beutilized. In most cases, from 0.01 to 0.1 phm of an organolithiuminitiator will be utilized with it being preferred to utilize 0.025 to0.07 phm of the organolithium initiator.

The multifunctional initiators which can be used include those preparedby reacting an organomonolithium compounded with a multivinylphosphineor with a multivinylsilane, such a reaction preferably being conductedin an inert diluent such as a hydrocarbon or a mixture of a hydrocarbonand a polar organic compound. The reaction between the multivinylsilaneor multivinylphosphine and the organomonolithium compound can result ina precipitate which can be solubilized if desired, by adding asolubilizing monomer such as a conjugated diene or monovinyl aromaticcompound, after reaction of the primary-components. Alternatively, thereaction can be conducted in the presence of a minor amount of thesolubilizing monomer. The relative amounts of the organomonolithiumcompound and the multivinylsilane or the multivinylphosphine preferablyshould be in the range of about 0.33 to 4 moles of organomonolithiumcompound per mole of vinyl groups present in the multivinylsilane ormultivinylphosphine employed. It should be noted that suchmultifunctional initiators are commonly used as mixtures of compoundsrather than as specific individual compounds.

Exemplary organomonolithium compounds include ethyllithium,isopropyllithium, n-butyllithium, secbutyllithium, tert-octyllithium,n-eicosyllithium, phenyllithium, 2-naphthyllithium,4-butylphenyllithium, 4-tolyllithium, 4-phenylbutyllithium,cyclohexyllithium, and the like.

Exemplary multivinylsilane compounds include tetravinylsilane,methyltrivinylsilane, diethyldivinylsilane, di-n-dodecyldivinylsilane,cyclohexyltrivinylsilane, phenyltrivinylsilane, benzyltrivinylsilane,(3-ethylcyclohexyl) (3-n-butylphenyl)divinylsilane, and the like.

Exemplary multivinylphosphine compounds include trivinylphosphine,methyldivinylphosphine, dodecyldivinylphosphine, phenyldivinylphosphine,cyclooctyldivinylphosphine, and the like.

Other multifunctional polymerization initiators can be prepared byutilizing an organomonolithium compound, further together with amultivinylaromatic compound and either a conjugated diene ormonovinylaromatic compound or both. These ingredients can be chargedinitially, usually in the presence of a hydrocarbon or a mixture of ahydrocarbon and a polar organic compound as a diluent. Alternatively, amultifunctional polymerization initiator can be prepared in a two-stepprocess by reacting the organomonolithium compound with a conjugateddiene or monovinyl aromatic compound additive and then adding themultivinyl aromatic compound. Any of the conjugated dienes or monovinylaromatic compounds described can be employed. The ratio of conjugateddiene or monovinyl aromatic compound additive employed preferably shouldbe in the range of about 2 to 15 moles of polymerizable compound permole of organolithium compound. The amount of multivinylaromaticcompound employed preferably should be in the range of about 0.05 to 2moles per mole of organomonolithium compound.

Exemplary multivinyl aromatic compounds include 1,2-divinylbenzene,1,3-divinylbenzene, 1,4-divinylbenzene, 1,2,4-trivinylbenzene,1,3-divinylnaphthalene, 1,8-divinylnaphthalene,1,3,5-trivinylnaphthalene, 2,4-divinylbiphenyl, 3,5,41-trivinylbiphenyl,m-diisopropenyl benzene, p-diisopropenyl benzene,1,3-divinyl-4,5,8-tributylnaphthalene, and the like. Divinyl aromatichydrocarbons containing up to 18 carbon atoms per molecule arepreferred, particularly divinylbenzene as either the ortho, meta, orpara isomer, and commercial divinylbenzene, which is a mixture of thethree isomers, and other compounds such as the ethylstyrenes, also isquite satisfactory.

Other types of multifunctional initiators can be employed such as thoseprepared by contacting a secor tert-organomonolithium compound with1,3-butadiene, at a ratio of about 2 to 4 moles of the organomonolithiumcompound per mole of the 1,3-butadiene, in the absence of added polarmaterial in this instance, with the contacting preferably beingconducted in an inert hydrocarbon diluent, though contacting without thediluent can be employed if desired.

Alternatively, specific organolithium compounds can be employed asinitiators, if desired, in the preparation of polymers in accordancewith the present invention. These can be represented by R(Li), wherein Rrepresents a hydrocarbyl radical containing from 1 to 20 carbon atoms,and wherein x is an integer of 1 to 4. Exemplary organolithium compoundsare methyllithium, isopropyllithium, n-butyllithium, sec-butyllithium,tert-octyllithium, n-decyllithium, phenyllithium, 1-naphthyllithium,4-butylphenyllithium, p-tolyllithium, 4-phenylbutyllithium,cyclohexyllithium, 4-butylcyclohexyllithium, 4-cyclohexylbutyllithium,dilithiomethane, 1,4-dilithiobutane, 1,10-dilithiodecane,1,20-dilithioeicosane, 1,4-dilithiocyclohexane, 1,4-dilithio-2-butane,1,8-dilithio-3-decene, 1,2-dilithio-1,8-diphenyloctane,1,4-dilithiobenzene, 1,4-dilithionaphthalene, 9,10-dilithioanthracene,1,2-dilithio-1,2-diphenylethane, 1,3,5-trilithiopentane,1,5,15-trilithioeicosane, 1,3,5-trilithiocyclohexane,1,3,5,8-tetralithiodecane, l,5,10,20-tetralithioeicosane,1,2,4,6-tetralithiocyclohexane, 4,4′-dilithiobiphenyl, and the like.

The organoaluminum compounds that can be utilized typically have thestructural formula:

in which R₁ is selected from the group consisting of alkyl groups(including cycloalkyl), aryl groups, alkaryl groups, arylalkyl groups,alkoxy groups, and hydrogen; R₂ and R₃ being selected from the groupconsisting of alkyl groups (including cycloalkyl), aryl groups, alkarylgroups, and arylalkyl groups. Some representative examples oforganoaluminum compounds that can be utilized are diethyl aluminumhydride, di-n-propyl aluminum hydride, di-n-butyl aluminum hydride,diisobutyl aluminum hydride, diphenyl aluminum hydride, di-p-tolylaluminum hydride, dibenzyl aluminum hydride, phenyl ethyl aluminumhydride, phenyl-n-propyl aluminum hydride, p-tolyl ethyl aluminumhydride, p-tolyl n-propyl aluminum hydride, p-tolyl isopropyl aluminumhydride, benzyl ethyl aluminum hydride, benzyl n-propyl aluminumhydride, and benzyl isopropyl aluminum hydride, diethylaluminumethoxide, diisobutylaluminum ethoxide, dipropylaluminum methoxide,trimethyl aluminum, triethyl aluminum, tri-n-propyl aluminum,triisopropyl aluminum, tri-n-butyl aluminum, triisobutyl aluminum,tripentyl aluminum, trihexyl aluminum, tricyclohexyl aluminum, trioctylaluminum, triphenyl aluminum, tri-p-tolyl aluminum, tribenzyl aluminum,ethyl diphenyl aluminum, ethyl di-p-tolyl aluminum, ethyl dibenzylaluminum, diethyl phenyl aluminum, diethyl p-tolyl aluminum, diethylbenzyl aluminum and other triorganoaluminum compounds. The preferredorganoaluminum compounds include triethyl aluminum (TEAL), tri-n-propylaluminum, triisobutyl aluminum (TIBAL), trihexyl aluminum and diisobutylaluminum hydride (DIBA-H).

The barium salts of cyclic alcohols that can be used can be mono-cyclic,bi-cyclic or tri-cyclic and can be aliphatic or aromatic. They can besubstituted with 1 to 5 hydrocarbon moieties and can also optionallycontain hetero-atoms. For instance, the barium salt of the cyclicalcohol can be a metal salt of a di-alkylated cyclohexanol, such as2-isopropyl-5-methylcyclohexanol or 2-t-butyl-5-methylcyclohexanol.These barium salts are preferred because they are soluble in hexane.Barium salts of disubstituted cyclohexanol are highly preferred becausethey are soluble in hexane. Barium mentholate is the most highlypreferred barium salt of a cyclic alcohol that can be employed in thepractice of this invention. Barium salts of thymol can also be utilized.The barium salt of the cyclic alcohol can be prepared by reacting thecyclic alcohol directly with the barium or another barium source, suchas barium hydride, in an aliphatic or aromatic solvent.

The lithium salts of cyclic alcohols that can be used can be used can bemono-cyclic, bi-cyclic or tri-cyclic and can be aliphatic or aromatic.They can be substituted with 1 to 5 hydrocarbon moieties and can alsooptionally contain hetero-atoms. For instance, the lithium salt of thecyclic alcohol can be a lithium salt of a di-alkylated cyclohexanol,such as 2-isopropyl-5-methylcyclohexanol or2-t-butyl-5-methylcyclohexanol. These lithium salts are preferredbecause they are soluble in hexane. Lithium salts of disubstitutedcyclohexanol are highly preferred because they are soluble in hexane.Lithium mentholate is the most highly preferred lithium salt of a cyclicalcohol that can be employed in the practice of this invention. Lithiumsalts of thymol can also be utilized. The lithium salt of the cyclicalcohol can be prepared by reacting the cyclic alcohol directly with thelithium or another lithium source, such as lithium hydride, in analiphatic or aromatic solvent.

The molar ratio of the organoaluminum compound to the organolithiumcompound will be within the range of about 0.3:1 to about 8:1. It willpreferably be within the range of about 0.5:1 to about 5:1 and will mostpreferably be within the range of about 1.2:1 to about 2:1. The molarratio of the barium salt of the cyclic alcohol to the organolithiumcompound will be within the range of about 0.1:1 to about 1.8:1. Themolar ratio of the barium salt of the cyclic alcohol to theorganolithium compound will preferably be within the range of about0.15:1 to about 1.2:1 and will most preferably be within the range ofabout 0.2:1 to about 0.6:1. The molar ratio of the lithium salt of thecyclic alcohol to the organolithium compound will be within the range ofabout 0.15:1 to about 4:1. The molar ratio of the lithium salt of thecyclic alcohol to the organolithium compound will preferably be withinthe range of about 0.25:1 to about 2.5:1 with ratios within the range ofabout 0.6:1 to about 1:1 being most preferred.

The polymerization temperature utilized can vary over a broadtemperature range of from about 20° C. to about 120° C. In most cases, atemperature within the range of about 40° C. to about 100° C. will beutilized. It is typically most preferred for the polymerizationtemperature to be within the range of about 60° C. to about 90° C. Lowerpolymerization temperatures generally result in higher polymer meltingpoints. However, the glass transition temperature of thetrans-1,4-polybutadiene does not change as a function of polymerizationtemperature. The pressure used will normally be sufficient to maintain asubstantially liquid phase under the conditions of the polymerizationreaction.

The polymerization is conducted for a length of time sufficient topermit substantially complete polymerization of monomers. In otherwords, the polymerization is normally carried out until high conversionsare attained. The polymerization can then be terminated using a standardtechnique. The polymerization can be terminated with a conventionalnoncoupling type of terminator, such as water, an acid, a lower alcohol,and the like or with a coupling agent. For instance, coupling agents canbe used in order to improve the cold flow characteristics of thetrans-1,4-polybutadiene rubber and rolling resistance of tires madetherefrom. It also leads to better processability and other beneficialproperties. A wide variety of compounds suitable for such purposes canbe employed. Some representative examples of suitable coupling agentsinclude: multivinylaromatic compounds, multiepoxides, multiisocyanates,multiimines, multialdehydes, multiketones, multihalides (such as tintetrachloride and silicon tetrachloride), multianhydrides, multiesterswhich are the esters of polyalcohols with monocarboxylic acids, and thediesters which are esters of monohydric alcohols with dicarboxylicacids, and the like.

After the copolymerization has been completed, thetrans-1.4-polybutadiene can be recovered from the organic solvent. Thetrans-1,4-polybutadiene can be recovered from the organic solvent andresidue by means such as decantation, filtration, centrification and thelike. It is often desirable to precipitate the trans-1,4-polybutadienefrom the organic solvent by the addition of lower alcohols containingfrom about 1 to about 4 carbon atoms to the polymer solution. Suitablelower alcohols for precipitation of the segmented polymer from thepolymer cement include methanol, ethanol, isopropyl alcohol,normalpropyl alcohol and t-butyl alcohol. The utilization of loweralcohols to precipitate the trans-1,4-polybutadiene from the polymercement also “kills” the living polymer by inactivating lithium endgroups. After the trans-1,4-polybutadiene is recovered from thesolution, steam stripping can be employed to reduce the level ofvolatile organic compounds in the segmented polymer.

This invention is illustrated by the following examples which are merelyfor the purpose of illustration and are not to be regarded as limitingthe scope of the invention or the manner in which it can be practiced.Unless specifically indicated otherwise, parts and percentages are givenby weight.

Example 1

In this experiment, 2000 g of a silica/amumina/molcular sieve driedpremix containing 18.3 weight percent 1,3-butadiene was charged into aone-gallon (3.8 liters) reactor. Then, 6.6 milliliters (ml) of a 0.2 Msolution of barium thymolate (BAT) in ethylbenzene, 3.4 ml of a 1.0 Msolution of menthol in hexanes, 6.6 ml of a 1.02 M solution ofn-butyllithium (n-BuLi) in hexanes and 6.2 ml of a 0.87 M solution oftriethylaluminum (TEA) were added to the reactor. The molar ratio of BATto menthol to n-BuLi to TEA was 1:2.5:5:4.

The polymerization was carried out at 90° C. for 3 hours. The GCanalysis of the residual monomer contained in the polymerization mixtureindicated that 93% monomer was consumed after the 3 hour polymerizationtime. The polymerization was continued for an additional 30 minutes andthen, two ml of a 1 M ethanol solution in hexanes was added to shortstopthe polymerization and polymer was removed from the reactor andstabilized with 1 phm of antioxidant. After evaporating hexanes, theresulting polymer was dried in a vaccum oven at 50° C.

The polybutdiene produced was determined to have a glass transitiontemperature (Tg) at -93° C. and a melting temperature (Tm) at 8.1° C. Itwas then determined to have a microstructure which contained 4 percent1,2-polybutadiene units, 20 percent cis-1,4-polybutadiene units, and 76%trans-1,4-polybutadiene units. The Mooney viscosity (ML-4) at 100° C.for this polymer was determined to be 34.

Example 2

The procedure described in Example 1 was utilized in this example exceptthat the ratio of BAT to menthol to n-BuLi to TEA ratio was changed to1:4:10:8. About 90% on monomer was consusmed in 90 minutes. Theresulting polymer had a glass transition temperature of -95° C. and amelting point of 9.5° C. It was also determined to have a microstructurewhich contained 5% 1,2-polybutadiene units, 20% cis-1,4-polybutadieneunits, and 75% trans-1,4-polybutadiene units.

Example 3

The procedure described in Example 1 was utilized in this example exceptthat the polymerization was carried out at 65° C. The resulting polymerhad a glass transition temperature -95° and a melting point of 9.3° C.It was also determined to have a microstructure which contained 4%1,2-polybutadiene units, 16% cis-l, 4-polybutadiene units and 80%trans-1,4-polybutadiene units.

Example 4

The procedure described in Example 1 was utilized in this example exceptthat lithium t-butoxide was used in place of menthol. The resultingpolymer had a glass transition temperature of -95° C. and a meltingpoint of -7.6° C. It was also determined to have a microstructure whichcontained 6% 1,2-polybutadiene units, 24% cis-1,4-polybutadiene units,and 70% trans-1,4-polybutadiene units.

Example 5

The procedure described in Example 1 was utilized in this example exceptthat barium 2-ethylhexoxide was used in place of BAT. The resultingpolymer had a glass transition temperature of -95° C. and a meltingpoint of -24° C. It was also determined to have a microstructure whichcontained 7% 1,2-polybutadiene units, 29% cis-1,4-polybutadiene units,and 64% trans-1, 4-polybutadiene units.

Example 6

The produce described in Example 1 was utilized in this example exceptthat barium tetrahydrofurfurlate was used in place of BAT. The resultingpolymer had a glass transition temperature of -95° C. and a meltingpoint of -10° C. It was also determined to have a microstructure whichcontained 6% 1,2-polybutadiene units, 24% cis-1,4-polybutadiene units,and 70% trans-1,4-polybutadiene units.

Example 7

The procedure described in Example 1 was utilized in this example exceptthat the polymerization was conducted at 65° C. and that menthol was notused as part of catalyst component. The resulting polymer had a glasstransition temperature of -91° C. and a melting point of 11° C. It wasalso determined to have a microstructure which contained 5%1,2-polybutadiene units, 19% cis-1,4-polybutadiene units, and 76%trans-1,4-polybutadiene units.

Example 8

The procedure described in Example 1 was utilized in this example exceptthat menthol was not used as part of catalyst component. The resultingpolymer had a glass transition temperature of -95° C. and a meltingpoint of -14° C. It was-also determined to have a microstructure whichcontained 6% 1,2-polybutadiene units, 22% cis-1,4-polybutadiene units,and 72% trans-1,4-polybutadiene units.

Example 9

The procedure described in Example 1 was utilized in this example exceptthat menthol was not used as part of catalyst component with bariummentholate (BAM) was used in place of barium thymolate (BAT). Theresulting polymer had a glass transition temperature of -95° C. and amelting point of -13° C. It was also determined to have a microstructurewhich contained 6% 1,2-polybutadiene units, 25% cis-1,4- polybutadieneunits, and 69% trans-1,4-polybutadiene units.

Example 10

The procedure described in Example 9 was utilized in this example exceptthat polymerization was carried out at 75° C. The resulting polymer hada glass transition temperature of -94° C and a melting point of 7.10° C.It was also determined to have a microstructure which contained 5%1,2-polybutadiene units, 19% cis-1,4-polybutadiene units, and 76%trans-1,4-polybutadiene units. The Mooney viscosity of this polymer at100° C. was determined to be 78.

Example 11

The procedure described in Example 1 was utilized in this example exceptthat dibutylmagnesium (Bu2 Mg) was used instead of n-BuLi and bariummentholate (BAM) was used in place of barium thymolate (BAT). The ratioof BAM to Bu₂ Mg to t-BuOLi to TEA was 1:10:4:4. About 70% monomerconversion was achieved after 6 hours of polymerization time at 90° C.The resulting polymer has a glass transition temperature of -95° C. anda melting point of 8.1° C.

Example 12

The procedure described in Example 7 was utilized in this example exceptthat 1.5 times as much of the BAT was used with the ratio of BAT ton-BuLi to TEA ratio being 1:10:9. The resulting polymer has a glasstransition temperature of -91° C. and a melting point of 44° C. It wasalso determined to have a microstructure which contained 3%1,2-polybutadiene units, 13% cis-1,4-polybutadiene units, and 84%trans-1,4-polybutadiene units.

Variations in the present invention are possible in light of thedescription of it provided herein. While certain representativeembodiments and details have been shown for the purpose of illustratingthe subject invention, it will be apparent to those skilled in this artthat various changes and modifications can be made therein withoutdeparting from the scope of the subject invention. It is, therefore, tobe understood that changes can be made in the particular embodimentsdescribed which will be within the full intended scope of the inventionas defined by the following appended claims.

What is claimed is:
 1. A process for synthesizingtrans-1,4-polybutadiene which comprises polymerizing 1,3-butadienemonomer in the presence of a catalyst system which is comprised of (a)an organolithium compound, (b) a barium compound selected from the groupconsisting of (i) a barium salt of a di-alkylated cyclohexanol, and (ii)barium thymolate, and (c) an orgnoaluminum compound.
 2. A process asspecified in claim 1 wherein the polymerization temperature is withinthe range of about 20° C. to about 120° C.
 3. A process as specified inclaim 1 wherein the polymerization temperature is within the range ofabout 40° C. to about 100° C.
 4. A process as specified in claim 1wherein the polymerization temperature is within the range of about 60°C. to about 90° C.
 5. A process as specified in claim 2 wherein themolar ratio of the organoaluminum compound to the organolithium compoundis within the range of about 0.3:1 to about 8:1.
 6. A process asspecified in claim 5, wherein the molar ratio of the barium compound tothe organoaluminum compound is within the range of about 0.1:1 to about1.8:1.
 7. A process as specified in claim 4 wherein the molar ratio ofthe organoaluminum compound to the organolithium compound is within therange of about 0.5:1 to about 5:1.
 8. A process as specified in claim 7wherein the molar ratio of the barium compound to the organolithiumcompound is within the range of about 0.15:1 to about 1.2:1.
 9. Aprocess as specified in claim 5 wherein the molar ratio of theorganoaluminum compound to the organolithium compound is within therange of about 1.2:1 to about 2:1.
 10. A process as specified in claim 9wherein the molar ratio of the barium compound to the organolithiumcompound is within the range of about 0.2:1 to about 0.6:1.
 11. Aprocess as specified in claim 6 wherein the organolithium compound is analkyl lithium compound.
 12. A process as specified in claim 11 whereinthe barium compound is barium mentholate.
 13. A process as specified inclaim 1 wherein the barium compound is barium thymolate.
 14. A processas specified in claim 13 wherein the 1,3-butadiene monomer ispolymerized in a hydrocarbon solvent.
 15. A process as specified inclaim 14 wherein the organolithium compound is an organomonolithiumcompound.
 16. A process as specified in claim 15 wherein theorganomonolithium compound is utilized at a level which is within therange of 0.01 phm to 1 phm.
 17. A process as specified in claim 16wherein the organoaluminum compound is selected from the groupconsisting of triethyl aluminum, tri-n-propyl aluminum, trisiobutylaluminum, trihexyl aluminum, and diisobutyl aluminum hydride.
 18. Aprocess as specified in claim 17 wherein the polymerization temperatureis within the range of about 40° C. to about 100° C.
 19. A process asspecified in claim 18 wherein the molar ratio of the organoaluminumcompound to the organolithium compound is within the range of about0.3:1 to about 8:1; and wherein the molar ratio of the barium compoundto the organolithium compound is within the range of about 0.15:1 toabout 1.2:1.